Process for improving fatigue life of metal



United States Patent 3,453,153 PROCESS FOR IMPROVING FATIGUE LIFE OF METAL Glenn W. Tulfuell, Warwick, N.Y., Charles J. Novak, Ringwood, N.J., and Stephen Floreen, Suffern, N.Y., assignors to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed July 25, 1966, Ser. No. 567,410

Int. Cl. C21d N18 US. 'Cl. 148-134 7 Claims ABSTRACT OF THE DISCLOSURE A process for improving the fatigue life of allows, notably maraging steels, which comprises heating the alloy within a temperature range of about 1050" F. to about 1325 F. for up to about 24 hours and then heating within the temperature range of about 1350 F. to about 1500 F. for up to a like period.

The present invention relates to heat treating alloys and more particularly to a heat treatment for improving the fatigue life of ferrous-base alloys, notably the maraging type of steels.

As is well known to those skilled in the art, failure of metal in service can take one or more of many forms. Failure through corrosive attack per se, inability to withstand an applied level of impact energy (impact fracture), and creep rupture are but illustrative. While failure due to static overloading can arise, a situation in which the applied stress exceeds the yield point of the material,

its occurrence is now rather uncommon due to advances achieved in engineering design. A somewhat more fre quent occurrence, however, is failure by way of fatigue, i.e., failure induced by the application of repetitive stress, the stress being less than the tensile strength of the material.

Unlike elastic failure of metal due to buckling, overstressing, creep, etc., an unfortunate consequence, generally speaking, of fatigue fracture is that there is little advance warning of impending failure. As to the causes thereof, investigations have shown that such failure is usually associated with a flaw in the metal, whether it be induced internally or externally and whether it be a notch, crevice, nick, crack, hole, etc. Under repetitive stress the fla'w eventually propagates to the point at which fracture is brought about. Lack of warning and the virtual impossibility, as a practical matter, of preventing and/or eliminating such flaws rather accentuate the difficulties involved. Thus, Where repetitive stress is to be encountered, e.g., axles, springs, aircraft structural components, etc., a serious problem of no little magnitude arises and considerable attention must be directed to such factors as design, processing (including heat treatment and fabrication), intended use, and not least, the characteristics of the material(s) under consideration.

As is generally acknowledged, the maraging steels, developed but a few years ago, added a new dimension to ferrous metallurgy. These steels ofiered an outstanding combination of many desired mechanical and physical properties theretofore not available, particularly exceptional ductility and toughness at exceedingly high levels of yield strength. While these characteristics generated an extensive amount of research, little, comparatively speaking, has been accomplished in respect of improving the fatigue life of such steels.

When properly processed and given what has'become a standard heat treatment, maraging steels such as those described in US. Patent No. 3,093,519 manifest ice fatigue properties of about the same order of magnitude as the carbon and low allow steels of comparable tensile strength. Thus, viewed in the light of knowledge acquired from the carbon and loW alloy steels, the results achieved and reported herein are rather surprising. Generally speaking, attempts by way of modified heat treatments to increase the fatigue life of carbon and low allow steels of similar strength levels have not met with much success. Usually an upper limit of fatigue strength was obtained and this value was rather dependent upon the tensile strength of the steel. Accordingly, though heat treatment altered tensile strength, it normally did not confer markedly enhanced fatigue life.

It has now been discovered that the fatigue life of maraging steels herein set forth can be increased many fold by subjecting the steels to a sequence of heat treating operations over special but limited temperature ranges. In this connection and conducive to a better understanding of the invention, some discussion of the background concerning prior heat treatments of the maraging steels is in order.

As is known to the ferrous metallurgist, it has become accepted practice to anneal wrought maraging steels at a temperature of about 1500 F. for about one hour prior to aging at a temperature of about 900 F. for about three hours. This is a standard maraging heat treatment employed commercially, although it is at times accompanied by an initial processing heat treatment about 1500 F. (Since the aging treatment heretofore employed remains unchanged herein, 800 F. to 1000" F., little need be said with regard thereto. Heat treatment before aging, however, is of the essence and it is this aspect to which further comments will be principally addressed.)

The aforementioned standard annealing temperature of 1500 F. aside, other temperatures have been proposed and/or investigated. In fact, in the above-identified US. Patent No. 3,093,519, a solution treatment of 1300 F. to 2000 F. is set forth, although the data therein is based on temperatures of 1500 F. and 1600 F. Experimental work has been conducted using both lower and higher annealing temperatures, e.g., 1200 F., 1250 F., 1400 F., and 1800 F. or higher. Also, there have been indications that the annealing temperature can be dispensed with. In any event, experimental data published regarding heat treating, for example, at 1200 F. and 1250 F., have established that substantial amounts of austenite form, the austenite being of such stability that it does not transform to martensite upon cooling to room temperature. This austenite in large quantities of 10% or more (and reverted austenite is included) greatly detracts from yield strength and the recommendation has been to avoid these temperatures. In respect of experimental data published concerning an annealing temperature of 1400 F. before aging, one reference source indicates an optimum combination of properties can be obtained whereas another report recommended using 1500 F. as opposed to 1400 F. because of better reduction in area (ductility) characteristics.

Annealing temperatures above 1600 F., generally speaking, have not added to the fund of attributes conferred by the 1500 F. to 1600 F. range, yield and ultimate tensile strengths usually being lower as a consequence of the higher temperature. However, where severe forming operations are to be applied subsequent to the anneal and prior to aging, temperatures beyond 1600 F. have been advocated. Too, as indicated herein, an initial processing treatment conducted at a temperature above 1500 F. and then followed by an anneal at, say, 1500 F. has been used to some extent.

To the foregoing should be added that cast maraging steels are usually annealed commercially at about 2100" 3 F. before aging at 900 F., although the utilization of additional heating between the initial solution treatment and aging steps has been recently proposed.

The above synopsis of prior maraging steel heat treatments, brief though it is, reflects that a fair amount of work has been conducted regarding the effect of annealing temperature. As to Why 1500 F. has become a standard, the more probable reasons would include (1) achieving a recrystallized structure and eliminating the structure obtained from prior working, (2) assuring a complete austenitic structure such that the austenite will transform to martensite on cooling, and (3) obtaining maximum ductibility. But notwithstanding what has transpired heretofore and insofar as we are aware, no heat treatment has been advanced in which fatigue life of maraging steels has been improved to any appreciable degree. And, it is the specific object of the present invention to enhance the fatigue life of maraging steels contemplated herein.

Generally speaking, the fatigue life of maraging steels containing about to 20%, e.g., to 19%, nickel; up to about 20%, e.g., 5% to 15%, cobalt; about 1% to 10%, e.g., 2.5% to 6%, molybdenum; up to 0.1%, e.g., up to 0.5%, carbon; up to 2%, e.g., up to 1%, titanium; up to 2%, e.g., up to 1%, aluminum; up to 3%, e.g., up to 1%, manganese; up to 3%, e.g., up to 1% silicon; and the balance essentially iron can be substantially enhanced by subjecting the steels (prior to aging) to a first stage temperature within the range of about 1050 F. to about 1325 F. for from a few minutes, e.g., 5 minutes, up to about 24 hours, and thereafter subjecting the steels to a second stage heating over the range of about 1350 F. to about 1500 F. for about one half hour to 24 hours, the longer periods being used with the lower temperature. As will be illustrated herein, fatigue life of various 18% nickel maraging steels, for example, can be improved by a factor of up to some 600% over that obtained with the standard treatment.

In carrying the invention into practice, it is beneficial to conduct the first stage heating over a temperature range of about 1100 F. to 1300 F. for about one half hour to two hours. Usually, a maximum period of two hours is satisfactory. In one sense, the use of an initial heat treatment over this temperature range is quite at odds with recommendations prior expressed. Put another way, reverted austenite is deliberately formed rather than intentionally avoided. While the exact theory which might explain the mechanism involved is not completely understood, it would appear the reverted austenite formed in the martensitic matrix (obtained upon cooling through the Ms-Mf range from hot working) acts in a manner whereby a very fine dispersion of discrete austenite particles are retained .upon cooling from the second stage heating (i.e., the austenitizing treatment). It is to be understood that while substantially all the austenite transforms on cooling from the second stage, it would appear that a small amount remains, to wit, less than the limits of detection provided by X-ray diffraction, i.e., less than 2%. In obtaining optimum fatigue life, it is most beneficial to water quench the steels from the second stage heating. This applies to all cooling operations herein. Air cooling, furnace cooling, etc., can, of course, be used but usually at a sacrifice in fatigue life.

The first stage treatment can be preceded by a preliminary annealing treatment well above 1500 F., e.g., 1500 F. to 2100 F. As is sometimes experienced, steels are improperly processed and this often adversely affects one or more desired characteristics. For example, where less than acceptable processing technique results in a poor structure from the consideration of, say, prior austenite grain boundaries, such a defect often can be obviated, at least substantially if not completely, by a preliminary solution treatment. But fatigue life is not improved-it is impaired, the degree of impairment increasing with the hrgner temperatures. While this adverse effect on fatigue lire brought about by high preliminary annealing temperatures is greatly minimized by the sequence of heat treating operations in accordance herewith, it is recommended that the preliminary treatment, if it must be applied at all, not exceed about 1700 F.

With regard to the second stage treatment, a most satisfactory temperature range is from about 1375 F. to 1450 F. for about one to four hours, four hours being used at 1375 F. And, for optimum results the maximum second stage temperature should not exceed about 1400 F., one hour being a suitable holding period. As will be shown herein, a temperature of 1400 F. imparts a markedly superior fatigue life than does, say, 1500 F.

In preparation of the steels, the use of high purity materials and vacuum processing are recommended although air melting practice can be employed. Ingots should be initially soaked and Worked at a temperature of about 2200 F. to 2300 F. A suitable final hot working temperature is over the range of 1700 F. to 1900 F. except for the finishing temperature which should be between about 1500" F. to 1600 F.

For the purpose of giving those skilled in the art a better appreciation of the advantages of the invention, commercially produced steels were tested, the compositions thereof being given in Table I:

TABLE I Percent Co Mo Ti Al O Mn Si Alloy No. Ni Fe Bal.

1 Bal.=Balance including impurities.

TABLE II Preliminary First Stage Second Stage Heat- Cool- Heat- 0001- Heat- Cooling ing ing ing ing ing Heat Treatment:

S.H.T None None 1,500 A.C None None 1,500 W.Q None 1,300 F.C.t0 1,500 W.Q

500 F., A.C. to R.I. None None 1,400 W.Q None None 1,300 W.Q None None 1,300 W.Q None None 1,700 W.Q None None 1,900 W.Q None None 2,000 W.Q None None 2,200 W.Q 1,500 W.Q. None 1,400 W.Q 1,650 W.Q. None 1,500 W.Q 2,000 W.Q. None 1,500 W.Q 2,000 W.Q,. 1,100 A.C 1,500 \V.Q 1,700 A.C. Non 1,400 A.C O 1,700 A.C. 1,100 AC 1,400 A.C

1 Degrees Fahrenheit, 1 hour.

2 Degrees Fahrenheit, 10 minutes.

2 Degrees Fahrenheit, 10 hours.

4 Degrees Fahrenheit, 2 hours.

N orn.S.H.T.=Standard Heat Treatment;

R.T.= Room Temperature; A.C.=Air Cooled (to R.T.); W.Q.=Water Quenehed (to R.T.); F.C.=Furnaee Cooled.

In connection with the test data reported in Table III, yield strength (Y.S.) and ultimate tensile strength (U.T.S.) are given in thousands of pounds per square inch (K s.i.) and the tensile elongation (EL) and reduction in area (R.A.) are given in percent. In respect of the fatigue life data, a maximum fiber stress of 130,000 p.s.i.

was used, the number of cycles being obtained using an R.R. Moore Rotating Beam Machine. Test data was obtained using smooth bar test section specimens.

The data presented in Tables II and III reflect the sharply enhanced fatigue life obtainable with heat treatments in accordance herewith. In respect of alloy No. 1, a more than five-fold improvement was effected using treatment B (a treatment within the invention) in comparison with that obtained using the standard treatment (S.H.T.), notwithstanding that the second stage heating of B was conducted at 15 00 F. As previously indicated herein, it has been found that a second stage temperature of less than 1500 F., e.g., 1400 F., is decidedly more effective. A similar increase in fatigue life was experienced with alloy No. 2 as shown by comparison of the standard treatment with treatment 0.

Mention should be made that a single anneal at 1400 F. prior to aging (treatment C) greatly increased fatigue life above that conferred by the conventional 1500 F. anneal. While fatigue life is improved with a 1300 F. treatment per se (no second stage treatment) as shown by treatments D and E, each resulted in a fatigue life strikingly less than B. Further, tensile strength was appreciably reduced with treatment D, a fact attributable to a substantial amount of reverted austenite. With the ten hour heating period of E, a greater percentage of austenite transformed to martensite on cooling-thus, higher strength. The standard treatment together with treatments A, C and F through I generally illustrate the loss of fatigue life attendant increase in temperature. A comparison is invited between treatments K and L. As hereinbefore set forth, it is sometimes desirable to use a high temperature preliminary treatment. With regard to treatment L, a 2000 F. temperature was used and fatigue life was considerably less in comparison with K (conducted at 1650 F.). Treatment M somewhat obviated the drastic impairment of fatigue life brought about by treatment L.

The data, treatments I and N, further reflect that fatigue life is also improved by a double anneal in which the steels first are heated to induce recrystallization and thereafter heated within a range above about 1350 F. and below 1500 F., e.g., 1375 F. to 1450 F. The recrysallization temperature is beneficially conducted within the range of 1500" F. to 1700" F. for one to four hours.

It is noteworthy to mention that heat treatments within the invention should not be confused with treatments heretofore proposed for metallurgically different purposes. For example, in various high nickel steels containing, say, 25% nickel and above, it has been found necessary subsequent to solution treatment to ausage the steels prior to aging. This ausaging treatment has been conducted at temperatures of about 1200 F. to 1300 F., the purpose being to precondition the steels such that upon cooling to room temperature, transformation to martensite is achieved. Thereafter the martensitic steel is aged, but in the absence of the ausaging steep the steels would be austenitic prior to aging. In sharp contrast thereto, steels contemplated herein do not require an ausaging treatment.

Also, as will be appreciated by those skilled in the art, in referring to the iron content of the steels as constituting the balance or balance essentially, it is to be understood that the presence of other elements is not excluded, such as those commonly present as incidental elements, e.g., deoxidizing and cleansing elements, and impurities ordinarily associated therewith in small amounts which do not adversely affect the basic characteristics of the steels. But elements such as sulfur, phosphorus, hydrogen and oxygen should be maintained at levels as low as is consistent with good commercial steelmaking practice. However, auxiliary hardening and/or strengthening elements can be present as follows: up to 1%, e.g., up to 0.4%, beryllium; up to 6%, e.g., up to 4%, copper; up to 10%, e.g., up to 7%, tungsten; up to 3%, e.g., up to 2%, columbium; up to 6%, e.g., up to 4%, vanadium; and up to 0.1% or 0.2% nitrogen. The total sum of the auxiliary elements should not exceed 10% and beneficially does not exceed 7%. Up to 8% chromium can be present but the total sum of nickel plus chromium should not exceed about 23%; otherwise, additional processing, for example, a cold treatment as by refrigeration and/ or cold Working, may be necessary to effect a satisfactory martensitic structure before application of the first stage treatment.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to Without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. For example, it is contemplated that'the fatigue life of various steels or alloys other than the maraging steels can be improved by the heat treatments in accordance herewith. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A process for enhancing the fatigue life of a wrought maraging steel, said steel having been previously hot worked and consisting essentially of about 10% to 20% nickel, up to 20% cobalt, from 1% to 10% molybdenum, up to 0.1 carbon, up to 2% each of titanium and aluminum, up to 3% each of manganese and silicon, up to 1% beryllium, up to 6% copper, up to 10% tungsten, up to 3% columbium, up to 6% vanadium, up to 0.2 nitrogen, the beryllium, copper, tungsten, columbian, vanadium and nitrogen not exceeding 10% in total, up to 8% chromium, the balance being iron, which comprises subjecting the steel to a first stage temperature within the range of about 1050 F. to about 1325 F. for a period up to about 24 hours, cooling the steel, and thereafter subjecting the steel to a second stage heating over the range of about 1350" F. to about 1500 F. for about one half hour to 24 hours.

2. The process as set forth in claim 1 in which the first stage temperature is within the range of about 1100 F. to about 1300 F. for about one half hour to two hours.

3. The process as set forth in claim 1 in which the second stage temperature is within the range of about 1375 F. to about 1450 F. for about one hour to four hours.

4. The process as set forth in claim 1 in which the first stage temperature is conducted within the range of about 1100" F. to about 1300 F. for about one half hour to two hours and the second stage heating within the range of about 1375 F. to about 1450 F. for about one hour to four hours.

5. The process as set forth in claim 4 in which the second stage temperature is about 1400 F.

6. The process as set forth in claim 1 in which the steel contains about 15% to 19% nickel, about 5% to 15 cobalt, about 2.5% to 6% molybdenum, up to about 0.05% carbon, up to about 1% each of titanium and aluminum, up to about 1% each of manganese and silicon,

up to 0.4% beryllium, up to 4% copper, up to 7% tungsten, up to 2% columbium, up to 4% vanadium, up to 0.1% nitrogen, the sum of the beryllium, copper, tungsten, columbium, vanadium and nitrogen not exceeding 7%, and up to 8% chromium, the sum of the chormium plus nickel not exceeding about 23%.

7. The process as set forth in claim 4 in which the steel contains about 15% to 20% nickel, about 5% to 15% cobalt, about 2.5% to 6% molybdenum, up to about 0.05% carbon, up to about 1% each of titanium and aluminum, up to about 1% each of manganese and silicon, the balance being essentially iron.

References Cited UNITED STATES PATENTS 3,093,518 -6/1963 Bieber 148l42 X 3,093,519 6/1963 Decker et 211. 3,131,097 4/1964 Mantel 148-143 X 3,210,224 10/1965 Argo 148142 3,341,372 9/1967 Sadowski 148142 US. Cl. X.R.

#050 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Itent No.Ji-53,l53 Dated lug X J 1959 wentofls) GLENN W.TUFFNELL, CHARLES J.NOVAK and STEPHEN FLOREEN It is certified that error appears in the above-identified patent \d that said Letters Patent are hereby corrected as shown below:

olumn 2, line 2, "allow read --alloy--.

olumn 2, line 28, "about" read --above-.

olumn 3, line 13, "ductibility" read --duct11ity--.

olumn 4, Table II, Heat Treatment 0, 1,700" read 1,700" and 1,100" read --*1,1o0-.

olumn t, Table II, Footnote, Degrees Fahrenheit, 10 hours." read Degrees Fahrenheit, 10 hours.

olumn 5, Table III, Heat Treatment A, "161 read --261--. olumn 6, line 1, "steep" read step--. olumn 6, line 47 (Claim 1, line 8) 0.2 nitrogen, read --O.2% nitrog olumn 6, line 48 (Claim 1, line 9) "columbian" read --columbium--.

A- SSALED N07 3 1% (SEAL) Attesu Eamranmewhmh I mom mm In W, no

Oomiasiom or Patent! 

